Volcanic eruptions are the most important natural cause of climate change, and they teach us many lessons about the climate system.  The cooling Earth experiences for a couple years after a big volcanic eruption, like that of Mt. Pinatubo in 1991, helps us calibrate the amount of warming we will suffer in the future from continued human emissions of greenhouse gases, such as carbon dioxide.  By filtering out the episodic responses to large eruptions, or the cumulative impacts of small eruptions, such as those of the past decade, the global warming signal becomes clear.

Plume from the Nabro volcanic eruption in Eritrea, the largest eruption in 20 years in terms of its stratospheric sulfur input. NASA Moderate Resolution Imaging Spectroradiometer (MODIS) image from 13 June 2011 (http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=50988).

Plume from the Nabro volcanic eruption in Eritrea, the largest eruption in 20 years in terms of its stratospheric sulfur input. NASA Moderate Resolution Imaging Spectroradiometer (MODIS) image from 13 June 2011 (http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=50988).

Large volcanic eruptions create a cloud of sulfuric acid droplets in the stratosphere, the layer of the atmosphere above the troposphere where we live.  Over the next couple years the sulfur falls out of the atmosphere.  The layers of acid that are preserved in the Greenland and Antarctic ice caps serve as a record of past eruptions, and allow us to understand how and when these eruptions cooled Earth.  In fact, we now understand that a series of large eruptions near the end of the 13th Century initiated the Little Ice Age, which only ended when humans started using the atmosphere as a sewer for the carbon dioxide resulting from burning coal, oil, and natural gas from the Industrial Revolution.

Noticing the cooling that occurs naturally from volcanic clouds, some have suggested that society create such a cloud permanently, with airplanes or hoses or artillery, to cool Earth and reduce or reverse global warming.  This idea is called geoengineering or solar radiation management.  While volcanic eruptions show that a stratospheric cloud would indeed cool the surface, reducing ice melt and sea level rise, they also make clear that a stratospheric aerosol cloud would produce ozone depletion, allowing more harmful ultraviolet radiation at the surface, reduce summer monsoon precipitation and even produce drought, produce rapid warming if geoengineering were suddenly stopped, reduce solar power, damage airplanes flying in the stratosphere, and degrade surface astronomical observations and remote sensing. This raises many issues about the wisdom of geoengineering, and there are many other reasons why geoengineering may be a bad idea, including that it could be used as a weapon or produce international conflict if different nations cannot agree on where to set the planetary thermostat.

The side of Eyjafjallajökull during a field trip for the AGU Chapman Conference on Volcanism and the Atmosphere, 13 June 2012. Photo by Alan Robock.

The side of Eyjafjallajökull during a field trip for the AGU Chapman Conference on Volcanism and the Atmosphere, 13 June 2012. Photo by Alan Robock.

There is another way that humans could produce a cloud in the stratosphere, made of black soot particles.  Following a nuclear war, fires in targeted cities and industrial areas would pump smoke into the stratosphere, where winds would spread it around the world, remain for more than a decade, destroy ozone, and make it dark, cold, and dry at Earth’s surface, killing crops and producing famine.  The use of only 100 nuclear weapons of the size that destroyed Hiroshima in 1945 could produce climate change unprecedented in recorded human history, which could produce significant decreases of agriculture in the main grain-growing regions of the world.  Nuclear winter theory should never be tested in the real world, but volcanic eruptions are a close analog of all these impacts, validating climate model calculations of the effects of nuclear war.

Humans can do nothing to prevent future volcanic eruptions.  But we can slow global warming and prevent nuclear war.  We know that a combination of conservation, more efficient use of energy, and rapid shift to solar and wind power will avert the worst future impacts of global warming. While the world has banned cluster munitions, land mines, biological weapons, and chemical weapons, the worst weapons of mass destruction, nuclear weapons, are still not banned.  They cannot be used, they do not serve as a deterrent, and their use would be suicide.  We can rid the world of nuclear weapons so we have the luxury of working to address global warming without the fear of global catastrophe.  For more information, please visit my website and join the International Campaign to Abolish Nuclear Weapons.

-Alan Robock is a Distinguished Professor of Climatology at Rutgers University, who has worked for his entire career on the climatic effects of volcanic eruptions, global warming, and nuclear weapons.  He served as a Peace Corps Volunteer in the Philippines in 1970-1972 and as a Congressional Science Fellow in 1986-1987.  He is a Fellow of the AGU and former President of the AGU Atmospheric Sciences Section.

Read more about this topic in Alan Robock’s recent article in Eos